Annual and semiannual variations of the location and intensity of large‐scale field‐aligned currents

Abstract

The present study examines seasonal variations of large‐scale field‐aligned current (FAC) systems in terms of the dipole tilt and clock angles. Magnetic field measurements from the DMSP F7 and F12‐F15 satellites are used. This data set consists of a total of ∼185,000 auroral oval crossings, out of which ∼121,000 crossings were selected for the present analysis. Focus is placed on the latitude at the demarcation between the region 2 (R2) and region 1 (R1) currents and the intensities of these currents. It is found that the dayside FAC moves poleward and equatorward in the summer and winter hemispheres, respectively, and the nightside FAC has the opposite seasonal dependence. In the midday sector the peak‐to‐peak variation of the FAC latitude over the entire range of the dipole tilt is ∼5°, whereas it is ∼4° around midnight. In the flank sectors the average FAC latitude is higher around the solstices than around the equinoxes irrespective of hemisphere. The corresponding dependence on the dipole clock angle can actually be found for almost all local time sectors, although the peak‐to‐peak variation of the expected semiannual variation, 2° around noon and <1° in other local time sectors, is smaller than that of the annual variation except for the flank sectors. A comparison with a model magnetic field strongly suggests that the dipole tilt effect on the magnetospheric configuration is the primary cause of the annual variation, whereas the semiannual variation is inferred to reflect the fact that geomagnetic activity tends to be higher around the equinoxes. The average dayside FAC intensity is larger in the summer hemisphere than in the winter hemisphere, which can be explained in terms of the seasonal variation of the ionospheric conductivity. The dayside R1 current intensity depends more strongly on the dipole tilt than the dayside R2 current intensity, and it changes by a factor of 2–3 over the entire range of the dipole tilt angle. In contrast, the annual variation of the nightside FAC intensity is more complicated, and the nightside R2 current seems to be more intense in the winter hemisphere than in the summer hemisphere. The dependence of the FAC intensity on the dipole clock angle is less significant especially for the R1 system. Nevertheless, the result suggests that the FAC tends to be more intense around the equinoxes, which is consistent with the semiannual variation of geomagnetic activity.

Seasonal and interplanetary magnetic field dependence of the field-aligned currents for both Northern and Southern Hemispheres

“…Ohtani et al (2005b) extended this analysis and showed that the dayside Region 1 current intensity varies by a factor of 2 to 3 over the entire dipole tilt angle range. They also showed that the seasonal variation of the nightside FACs was more complicated with the nightside Region 2 current intensity more intense during winter than in summer.…”

“…The seasonal differences in the FAC patterns might suggest effects related to the Earth's dipole tilt angle, location of the OCFLB or effects related to solar illumination and the distribution of ionospheric conductivity over the polar regions. Ohtani et al (2005b) suggested that the seasonal change in the geomagnetic field configuration may explain the poleward shift in the location of the dayside FACs in summer compared with winter since the dayside R1 currents are thought to be located near the OCFLB. In order to investigate this we have overlayed on each FAC map an average OCFLB estimated from the Tsyganenko and Sitnov (2005) TS05 magnetic field model for the mean IMF and solar wind conditions associated with the events used in constructing each FAC map.…”

“…These conflicting results were attributed to different definitions of FAC location. Ohtani et al (2005b) compared their FAC locations with the Tsyganenko (1996) magnetic field model and concluded that the effect of the dipole tilt angle was the main cause of the annual variation in FAC latitude. Ohtani et al also examined the dawn-dusk seasonal dependence.…”

“…However, the same analysis of Magsat data showed that the dawn-dusk cross polar cap distance of the Region 1/Region 2 currents had little seasonal dependence. Ohtani et al (2005b) examined the seasonal change in the location of dayside FACs using DMSP data and showed a relative difference of up to 5 • between the poleward summer and equatorward winter locations. In contrast to the Oersted results of Christiansen et al (2002), Ohtani et al (2005b) found that the nightside FACs moved equatorward in summer and poleward in winter with a maximum shift of 4 • .…”

“…Region 0 currents were initially named the cusp current (Iijima and Potemra, 1976a) and have also been called the "traditional cusp current" and the "mantle current" due to possible collocation with cusp particle precipitation. Following Ohtani et al (2005b), we use the term "Region 0". The distribution of Region 0 and dayside Region 1 currents strongly depends on IMF B y (Iijima et al, 1978;Erlandson et al, 1988).…”